Remote convenience systems are known in the art. Such remote convenience systems permit remote control of certain functions. One example type of a remote convenience system is for remotely controlling vehicle functions. Other example types of remote convenience systems include garage door opener systems and entry light activation systems.
Focusing now on the remote convenience vehicle systems, examples of remotely controlled functions include locking and unlocking of one or more vehicle doors. A remote convenience vehicle system that permits remote locking and unlocking functions is commonly referred to as a remote keyless entry ("RKE") system.
Such remote convenience vehicle systems may provide for control of other vehicle functions. For example, remote engine start and remote door open functions are known. Another example of a remote convenience function is a vehicle locator function. For the vehicle locator function, the vehicle horn is actuated to emit a horn chirp and/or the headlights of the vehicle flash to allow a person to quickly locate their car within a crowded parking lot.
Known remote convenience vehicle systems include a receiver mounted in an associated vehicle and at least one portable hand-held transmitter located remote from the receiver. The receiver has a memory that stores one or more security codes, each of which is associated with a transmitter that is authorized to cooperate with the receiver mounted in the vehicle. Each transmitter is provided with one or more manually actuatable switches. Each switch is associated with a vehicle control function to be performed.
Within each transmitter, circuitry is provided that is responsive to switch actuation to transmit a message in the form of a digital signal. The transmitted signal includes the appropriate security code and is intended for reception by the receiver. Upon reception of the signal by the receiver, the security code portion of the received signal is compared against a stored security code by an actuation controller. If the security codes match, the received command message is decoded by the actuation controller. In turn, the controller directs performance of the requested function.
The portable transmitters operate in the ultrahigh frequency ("UHF") portion of the radio frequency ("RF") spectrum. Specifically, the remote transmitters operate in the portion of the RF spectrum that is allocated by the Federal Communications Commission ("FCC") for unlicensed transmission devices. FCC regulations stipulate that such unlicensed devices can not have a transmitted signal strength that exceeds a stipulated maximum value.
It is desirable to have a system that provides consistent performance within a certain range. It is also often desirable to accomplish remote control performance of certain functions at a longest possible distance.
As mentioned above, FCC regulations prevent a direct, overall increase in the transmitted signal strength. However, FCC regulations permit an increased instantaneous signal strength of a transmitted signal based upon an average strength within a transmission window. One approach to maintaining a predetermined average strength is to associate a certain amount of "dead time" with each signal transmission. In one example, for a 100-millisecond period, the dead time can be 45 milliseconds or greater. Thus, transmission only occurs during time portions that add-up to 55 milliseconds (or less) within the 100-millisecond period. In one example, the transmitter emits the signal for a short time period (e.g., 22.5 milliseconds), idles for a short time period (e.g., 27.5 milliseconds), and then repeats the transmission/idle sequence.
Turning now to the receiver within the remote convenience system, the receiver must have a very low power draw. One reason for the low power requirement is that the receiver relies on power provided by a battery of the vehicle within which the receiver is located. If the vehicle is inactive for a very long time period while the receiver is "ON", sufficient power could be drained from the battery to effect other operational functions of the vehicle (i.e., starting of the vehicle after the long idle period). One approach to reducing the power consumption of the receiver is to turn the receiver "OFF" for a period of time.
A typical ON/OFF schedule for a remote convenience system receiver is such that the receiver is ON less than 10 percent. In one example, the receiver is ON for 50 milliseconds and OFF for 450 milliseconds. However, in order for the receiver to receive a signal, the receiver must be "ON" (i.e., seeking or "listening" for such a signal) during at least one of the transmissions of the signal. It is possible for many transmission signals to occur while the receiver is OFF.
A receiver in a remote convenience system must also respond to a transmitted signal without excessive delay. A typical response time is desired to be less than one-half second. In other words, when a vehicle operator actuates a button on a hand-held transmitter, the operator expects the function (i.e., unlock the doors) to be performed within a relatively quick time period.
Circuitry within a known receiver has a start-up time period (e.g., stabilization of bias points and setting of threshold levels) when the receiver circuitry is turned "ON" after the long OFF time period. During the start-up period, the power is ON, but the receiver is not yet fully operational to detect/receive signals. A typical start-up time for a receiver takes approximately 20 milliseconds.
A worse case scenario for response time occurs when a transmitter operator actuates the transmitter at the beginning of a receiver "OFF" period (e.g., a 450-millisecond OFF period). During the worse case scenario, all of the signals transmitted during the receiver OFF time are not received. Also, during the receiver start-up time (20 milliseconds), the transmitted signals are not received. Thus, only 30 milliseconds remain within the half-second period within which the receiver can respond and still be within the expected half-second time frame for function performance after transmitter actuation. If the transmitted signal has a "dead time" that is greater than 30 milliseconds and that dead-time overlays the final 30 milliseconds of the half-second period (recalling that dead-time is used to increase transmission signal strength while maintaining an average transmission signal strength), the transmitted signal is not be received (i.e., picked-up or "caught") by the receiver and the one-half second time period is exceeded.
Accordingly, it should now be appreciated that the desire to have a relatively large dead time associated with transmission of signals and the desire to have a relatively low power consumption for the receiver pose a conflict when taken in the context of the desired response time window.